The invention described herein relates to nuclear reactors and more particularly a method and apparatus for protecting the core of a reactor under conditions of a break in the inlet piping which causes loss of coolant flow through the reactor core.
During the course of operation of a nuclear reactor, coolant, usually water, is pumped through the reactor to a steam generator in a closed primary loop. As the pumped coolant flows upwardly through the reactor core, it absorbs heat from fuel rods and the thusly heated coolant is then circulated to the steam generator which transfers the heat to a secondary circuit used for driving a turbine-generator. In commercial size reactors, the coolant is circulated at a pressure of about 2250 psia and its temperature at the reactor outlet is about 610.degree. F. Should the loop piping leading to the reactor inlet nozzle suffer a sufficiently large rupture or break, all coolant normally flowing toward the reactor will be diverted out of the pipe break and thus never reach the reactor inlet. Concurrently, the prevailing pressure in the reactor causes coolant therein to flow in a reverse direction through the inlet nozzle thereby evacuating the reactor of coolant because there is no impediment to flow between the reactor and the break in the inlet piping. It is known that under these circumstances, the reactor will purge itself of substantially all coolant within a short time period, and lack of coolant in the core then causes the fuel rods to reach high temperature levels. This temperature rise occurs even though the reactor control rods are immediately lowered to a fully inserted position in the core. Such temperature rise therefore is attributable both to a slight continuation of the fission process and to residual heat stored in fuel rods containing the fuel during normal reactor operation.
To safeguard the reactor against these undesirable conditions, the Atomic Energy Commission Reactor Design Criteria requires that manufacturers provide a source of emergency coolant, usually borated water, which promptly can be injected into the reactor core when the reactor pressure drops below a predetermined minimum. Various methods and emergency core cooling designs have been developed to assure delivery of such borated water to the reactor. However, even through such emergency systems are available, in the circumstances of a major pipe break, after the emergency coolant is injected into the reactor core, it immediately will follow the path of the main coolant stream and flow reversely through the inlet nozzle toward the pipe break because there are no restrictions or other devices present to preclude such outward flow. Further, because the fuel rod surfaces increase in temperature at an extremely high rate in the absence of coolant, and since the reaction times for achieving injecton of borated water into the reactor core is relatively long, the emergency core cooling systems thus devised are of complex design, are shrouded in high technology and involve high manufacturing and installation costs.
The major problem therefore which has always confronted reactor designers in providing safeguards against loss of coolant accidents, is how to retain a sufficient volume of primary and emergency coolant in the core and for a time period sufficient to achieve cool-down of the core fuel rods and reactor internal structural members to safe temperature levels.